feature: rimuc: `flex` layout tweaks and set `box-sizing` to `border-box` to stop padding growing containers (https://stackoverflow.com/questions/4767010/why-does-css-padding-increase-size-of-element).
feature: rimuc: `flex` layout tweaks and set `box-sizing` to `border-box` to stop padding growing containers (https://stackoverflow.com/questions/4767010/why-does-css-padding-increase-size-of-element).

srackham/rimu